Calibration of a gas sensor using data from a graph

Python Help
1

Newbie here.

I currently have a gas sensor connected to a raspberry pi. The sensor reading, however, is affected by temperature. I’d like to account for the change in order to get a more accurate reading. The manufacturer of the gas sensor has a graph which shows the temperature-dependent variation (attached below). I have a DH22 temperature sensor, and at first thought of just using a case statement or a bunch of IF statements to do the calibration, but I was wondering if there isn’t a better way of going about this?

The graph is non-linear which makes it a bit trickier. Thanks for your help.

ethylene graph
ethylene graph1163×824 106 KB

2

Do you have the polynomial for the fitted function? If so, just use that to calculate the correction.

If you only have the point data, then interpolate on that. Here’s a super simplistic (untested :grin:) 3-point example that should give you some ideas:

def correction(temp):
    X = -40.00,  0.00, 40.00
    Y =  -0.61, -0.41,  0.78
    for x, y in zip(X, Y):
        if temp < x: continue
        return y
    return 0.0  # Out of range.
3

You could try a log-log plot, i.e. if you have y=f(x) then plot log(y) vs log(x). This should make the plot a straight line. You can calculate the best fit for that straight line.

4

Adding a few datapoints estimated by eye, to the calculator on this site yields this info:

Fitted coefficients:
a = -0.3998
b = 0.01712
c = 0.0003258
• Quadratic model:
y = -0.3998 + 0.01712x + 0.0003258x2
• Coefficient of determination R²:
R²= 0.9961

This leads to a function something like:

def correction(temp: float) -> float:
    delta = -0.3998 + 0.01712 * temp + 0.0003258 * temp * temp
    return temp + delta

Details on my fit-by-eye

for t in range(-40, 60, 10):
    print(f"{t = :+3} delta = {correction(t) - t:+4.2f}")

_printed = '''
t = -40 delta = -0.56
t = -30 delta = -0.62
t = -20 delta = -0.61
t = -10 delta = -0.54
t =  +0 delta = -0.40
t = +10 delta = -0.20
t = +20 delta = +0.07
t = +30 delta = +0.41
t = +40 delta = +0.81
t = +50 delta = +1.27
'''